American Power Conversion 400, 600, 250 user manual Load transfer switch, Battery charger, Inverter

4.0 Principles of operation

4.2 Load transfer switch

The load transfer switch is actually an electro-dynamic relay which serves to rapidly transfer your computer equipment (load) from the utility to the UPS’s alternate power source in the event of a utility failure. When the utility is restored to within safe limits, the switch acts to re-transfer the load to the utility. Except for the user control switches, the transfer switch is the only moving part in the UPS. The time required for the relay to transfer your load to either power source is much, much faster than that which is required by any modern computer or computer peripheral device.

4.3 Battery charger

The UPS’s battery charger converts the alternating current (AC) supplied by the utility to a direct current (DC) which is compatible with the battery. The charger maintains the battery at a constant voltage to ensure that the battery will have the capacity to support your load as often as possible. This charging method, known as “float” charging, provides maximum battery service life and minimal internal heating. The battery is charged at an accelerated rate as required whenever the UPS is plugged in and switched on. When switched off, the battery is "trickle charged" in order to maintain full capacity.

4.4 Battery

The UPS’s battery is an energy source much like the battery in an automobile. Also, like most automobile batteries, the UPS’s battery is a modern maintenance free lead-acid type; it is sealed and leakproof. The battery has a typical service life of 3 to 6 years. The service life is extended when the UPS is kept in a cool environment, below 30°C or 86°F.

4.5 Inverter

The UPS must convert the battery’s energy into a form that your computer equipment can rely upon during a utility failure. This is the job of the UPS’s inverter. The UPS converts the battery’s DC to AC using solid state devices (transistors), controlled using a technique known technically as “pulse width modulation”. This technique is highly efficient which means that little battery power is wasted in the conversion process. Hence, your equipment can run for long periods from the UPS before the battery’s capacity is spent.

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